Miriam Carey, the woman who was shot dead by police after a high-speed chase that ended with the shutdown of the U.S. Capitol Thursday, likely had a related pre-existing condition that led to postpartum psychosis, according to one psychiatric expert.

Dr. Igor Galynker, director of the Family Center for Bipolar Disorder at Beth Israel Medical Center of the Mount Sinai Health System in New York City, said that going by news reports, Carey "most likely" had underlying biopolar disorder or schizophrenia, or a condition that is "halfway in between" -- schizoaffective disorder.

According to sources, Carey was on medication for unknown mental illness and had a family history of schizophrenia. Her family has also reported that she had suffered from postpartum depression.

Police had been called repeatedly to Carey's Stamford, Conn., apartment after the birth of her now 1-year-old daughter, Erica, after her boyfriend reported she was emotionally disturbed and that he suspected her of abuse and neglect of their daughter.

Galynker suggested that Carey's behavior did not look like postpartum depression, which is "relatively common and more like the blues." It does not include manic behavior or delusions, he said.

ABC News' sources revealed that Carey had started to show signs of mental illness around September 2012, and had a history of delusions and irrational behavior.

She reportedly told the father of her child and 54-year-old boyfriend at the time that she was the "prophet of Stamford" and that President Obama had placed the city on lockdown and had placed her residence under electronic surveillance, which was being fed live to all national news outlets.

Dr. Dost Ongur, chief of the psychotic disorders division at McLean Hospital in Boston, said that there is "no limit" to how long postpartum depression syndromes can last.

"It is not unheard of for symptoms to persist for a year after giving birth," he told ABCNews.com in an email.

But Galynker said that postpartum psychosis looks nothing like postpartum depression. Postpartum psychosis is associated with paranoia and delusions, said Galynker.

"[Carey's] main symptoms were not depression but were paranoia and delusions," he said. "But it is all unclear because no one has looked at her medical records. What we have seen is close to a year of psychotic episodes and hospitalizations and [encounters] with police."

Galynker said that quite often, postpartum psychosis is associated with mania, "which manifests itself as either euphoria or irritability," he said. "If she was irritable or neglectful or dysphoric or in a mixed state of mania and depression, those quite often come with psychosis.

"[Carey] may have been never been treated because of confidentiality issues or maybe family did not know about it," said Galynker. "Because she was pregnant, she may have gone off medications so as not cause birth defects. After delivery, she may have developed psychosis and continued on or off the medication for close to a year."

But Dr. Stephen M. Strakowski, professor and chairman of psychiatry and behavioral neuroscience at the University of Cincinnati, told ABCNews.com that postpartum mania could last for several months and the onset may be gradual. Drug and alcohol problems can also be associated with psychotic symptoms.

"Just to be clear, psychosis and depression are not mutually exclusive, and in fact, psychosis can be caused by depression, so it could still be postpartum depression," he said. "Family history is not always accurate, and if she is African-American, there is a high risk of misdiagnosis of schizophrenia when it's a mood disorder."

The patient can develop "frank psychosis, cognitive impairment and grossly disorganized behavior that represent a complete change from previous functioning," according to the study.

"These perturbations, in combination with lapsed insight into her illness and symptoms, can lead to devastating consequences in which the safety and well-being of the affected mother and her offspring are jeopardized," wrote study authors Dorothy Sit, Anthony J. Rothschild and Katherine L. Wisner.